Interpretive Summary: Current agricultural practices for row crops, such as corn and soybean, have resulted in increased nitrate concentrations in surface and ground water sources of drinking water. The use of winter cover crops is a management practice, which can reduce nitrate leaching and subsequent contamination of water supplies. Cover crops are plants that "cover" the soil surface and protect it from erosion. Winter cover crops usually grow between the fall harvest of one crop and the spring planting of the next crop. Cover crops reduce nitrate leaching by taking up nitrate during growth and by immobilizing nitrate during decomposition of their residues. An understanding of the decomposition process of cover crop roots and their effects on nitrogen availability is needed to determine if the nitrogen taken up by winter cover crops is available to the following crop. This will allow the development of cover crop management systems that reduce leaching and improve nitrogen-use efficiency. We conducted two controlled environment experiments to measure and compare decomposition rates of oat and rye roots mixed with soil or grown in place and to examine the effect of oat and rye root residues on soil nitrogen availability. We found that nitrogen was released from oat and rye roots very slowly during decomposition and may not become available to the following corn crop if the cover crops are killed in the spring right before planting. Additionally, we found that oat roots decomposed more rapidly than rye roots. The impact of this research will be the development of cover crop management practices that will reduce nitrate contamination of water supplies, will make more nitrate available to crops, and will make cover crops more practical and beneficial for farmers to utilize.

Technical Abstract:
A better understanding of decomposition and mineralization of cover crop roots is needed to synchronize root N mineralization with cover crop uptake. Two experiments were conducted in a controlled environment to measure and compare decomposition of oat and rye root residues, and to examine the effect of decomposition of these root residues on net soil N mineralization. In the first, oat and rye roots were mixed with soil and in the second, roots were grown in situ. Decomposition, denitrification, soil N0**3, and soil NH**4 were measured over 112 days to determine net mineralized N. Soil respiration and C and N contained in roots and coarse soil organic matter were measured to determine decomposition. All treatments in both experiments showed an increase in net mineralized N during the first 56 d. After 56 d, net mineralized N in the control remained relatively constant, whereas mineral N continued to accumulate in the treatments with root residues. No differences were detected in net N mineralization of the rye and oat root treatments. Differences in C mineralization, however, were observed as a function of both plant species and placement. Roots mixed with soil had high respiration rates during the first three days and there were no differences between oat and rye root treatments. In the roots in situ experiment, however, respiration peaked for oat roots at day 12 and for rye roots at day 33. Oat roots also decomposed and mineralized N more rapidly throughout the experiment. Based on these results we predict that less than 50% of the root N of a spring-killed small grain cover crop will become available to the following crop.